1. Field of the Invention
THIS INVENTION relates to a process of and an installation for use in making an extruded sintered ceramic artifact.
2. Brief Summary of the Invention
According to the invention there is provided a process for making an extruded sintered ceramic product, which process includes the steps of:
extruding an elongated moist profile, which profile comprises a moist ceramic composition or a moist precursor thereof; PA1 drying the extruded profile by subjecting it to microwave radiation having a frequency of 0.3.times.10.sup.9 -1.times.10.sup.10 Hz to cause the profile to become progressively drier in the direction in which it is extruded; and PA1 supporting the extruded profile during the extrusion and drying to provide a dried profile, PA1 the extruded profile being supported at a position where it has been at least partially dried by the microwave radiation. PA1 an extrusion means for extruding a profile of a moist mixture comprising a ceramic composition or precursor thereof; PA1 a support means for supporting the extruded profile; and PA1 at least one microwave radiation source operable to direct microwave radiation at at least part of the profile, for drying the profile, the support means being arranged to support the extruded profile at a position where the profile has at least partially been dried by microwave radiation from the microwave radiation source.
Supporting the profile will comprise applying a supporting force to the profile to prevent it from sagging or deforming unacceptably under gravity, the supporting force being applied to the profile at a position or positions where the profile has been dried sufficiently by the microwave radiation to receive such force without said unacceptable sagging or deforming. The supporting force is preferably applied to the profile at a position or positions removed or spaced from the microwave radiation.
The extruding may be in a downward direction, preferably in a vertically downward direction. The supporting in this case will thus be at a position below that where the extrusion takes place, and also below the position at which the profile is dried by the microwave radiation, to obtain a substantially straight dried extrusion.
Instead, the extrusion may be in an upward direction and the supporting may be at a position above the extrusion position. The supporting may be accompanied by displacing of the profile such as to cause a desired deformation of the profile, as it dries, eg so that the dried profile is curved along its length and/or has a differently shaped cross-section from that of the moist profile.
In each case the process of the invention may include the optional step of sintering the dried profile; and the process may be carried out continuously or batchwise.
As the extruding takes place, supporting the profile may be by frictionally engaging it by means of support means. In other words, when the extrusion is vertically downwardly, supporting the profile may be by frictionally engaging it, eg by means of support means in the form of one or more pairs of profiled rollers, whose rotation is synchronized with the extrusion, or by gripping it with releasable support means such as two or more pneumatic travelling chucks, conveniently arranged in pairs, which are moved synchronously with the rate of extrusion of the profile, said rotation or movement taking into account dimensional changes in the profile caused by the microwave radiation. Gripping the profile may be at a plurality of positions spaced along its length by a plurality of said releasable support means, each of which successively engages the profile at a high level, then descends synchronously with the profile, then releases the profile at a low level and then re-engages the profile at said high level. Preferably, gripping of the profile takes place at a position or positions where the profile has been dried. The profile may have a leading end which is extruded into contact with a support, the leading end resting on the support and the support descending synchronously with the profile.
The extrusion is preferably continuous. More particularly, the extrusion may be continuous, the drying taking place continuously while the profile is extruded, and the profile being supported continuously while it is extruded.
In a particular embodiment of the process of the present invention, the profile may be subjected to a progressively reducing microwave power input along its length during extrusion thereof, the rate of power reduction also preferably reducing progressively. While the rate of microwave power input, and the rate of reduction thereof, may reduce continuously, they preferably reduce stepwise. For example, the profile may, immediately after being extruded, be subjected to a relatively high microwave power input for a relatively short period, by passing it through a short zone of high microwave intensity, the profile then being subjected to an intermediate microwave power input for an intermediate period, by passing it through an intermediate zone of intermediate length and of intermediate microwave intensity, and the profile being finally subjected to a low microwave power input for a long period, by passing it through a relatively long zone of relatively low microwave intensity. In this context short and long with regard to length and period are relative, respectively, to the intermediate length and intermediate period, and high and low with regard to power input and intensity are relative, respectively, to the intermediate power input and intermediate intensity.
The process may include the step of intermittently and successively separating lowermost portions of the dried profile from the remainder thereof, after they have been subjected to the microwave radiation. The separation may be carried out by means of a vertically reciprocable cutting device, such as a flying saw, eg a diamond saw, laser beam or water jet, arranged to descend intermittently and synchronously with the profile, below the position where the profile is supported, the separation being carried out while the cutting device is descending. In other words, the separation may be carried out by means of a vertically reciprocating cutting device arranged to descend intermittently and synchronously with the profile, the separation being by cutting each said portion of the profile in a plane transverse to its length while the cutting device descends synchronously with the profile.
Thus, two or more vertically reciprocable support means such as travelling chucks may be provided for supporting the profile, a further vertically reciprocable support means may be provided for carrying a cutting device and one or more vertically reciprocable support means may be provided for gripping and moving cut portions down and out of the way of the descending profile, eg to a subsequent cooling stage or to a subsequent sintering stage and optionally thereafter. Preferably the profile, after extrusion, is supported so that it is subjected to tensile and compressive stresses which are no greater than those capable of being withstood by said profile, bearing in mind its strength and other physical characteristics, at the various stages of the microwave heating.
The profile may be subjected merely to low-temperature drying, by being heated to a maximum temperature of between 100-700.degree. C., preferably 100-500.degree. C., eg 120-200.degree. C., by the microwave radiation, before the portions are separated therefrom. It will be appreciated that suitable temperatures for drying, calcining and sintering of the profile will be dependent upon the composition of said profile. In one embodiment the profile is subjected to drying followed by microwave radiation whereby it is at least calcined by said microwave radiation, being heated thereby to a temperature of 700-1200.degree. C., eg 1050.degree. C., before the separation, the separation optionally taking place after some cooling of the profile.
While subjecting the profile to the microwave radiation may be such as to sinter the profile, eg by means of microwave radiation, before the portions are separated therefrom, in an alternative embodiment of the process the portions are separated after the profile has been dried, and optionally calcined, by the microwave radiation, but before the sintering takes place, the separated portions then being sintered, eg by subjecting them to further microwave radiation or infrared heating elsewhere. The sintering may be carried out by lowering each portion, while supporting it so that it extends vertically, and subjecting it to the further microwave radiation as it is lowered. Routine experimentation will be necessary to determine the optimum sintering temperatures depending on the composition of the portions. In one embodiment of the invention sintering may involve heating the portions (or the profile before it is severed if severing takes place after sintering) to a temperature of &gt;1200.degree. C., preferably &gt;1350.degree. C. and more preferably &gt;1550.degree. C., eg 1600-2400.degree. C. and typically 1600-1800.degree. C.
As mentioned above, the process may include the steps of calcining the dried profile and sintering said profile, the calcining and sintering being by subjecting the profile to further microwave radiation, to produce a ceramic product. Preferably, the calcining is carried out at a temperature of 700-1200.degree. C., the sintering being carried out at a temperature of 1600-2400.degree. C. The calcining and sintering may be carried out while the profile is being lowered and while it is being supported so that it extends vertically. The calcining and sintering, as described previously, may be carried out on portions of the profile which have been separated from the remainder thereof.
The rate of extrusion and the rate of heating of the profile should be matched in order to achieve a product of desired quality. Similarly, ambient atmospheric conditions surrounding the profile and/or portion should also be controlled with regard to circulation, humidity, temperature, composition, or the like.
More particularly, the profile may be extruded downwardly through a die into a microwave cavity. The extrusion may be by means of a ram or screw extruder. The cavity may be vertically elongated. In this case the cavity may be provided with a plurality of microwave sources, at least some of which are adjustable as regards their microwave output, and which may be both vertically and circumferentially spaced from each other around a central zone in the cavity down which the profile passes.
Said sources may be adjustable as to the frequency/wavelength of the microwave radiation produced thereby, as to the intensity/amplitude or power of the microwave radiation produced thereby, as to the direction of propagation of the microwave radiation and/or concentration or compression thereof into more or less discrete beams, etc. Thus, for example, the microwave cavity may be of the travelling wave type, the dynamically tuned standing wave type, the meander- or serpentine type, the multimode type to enable different overlapping standing waves to be generated, or a suitable hybrid type of two or more of the aforegoing.
For profiles of large cross-section, microwave cavities of the multimode type have been found to be suitable. The sintering may take place in a microwave cavity similar to that used for the drying.
As drying, calcining and sintering take place, the moisture content and structure/composition of the material of the profile or portion will change progressively, leading to progressive changes in the microwave absorbency or loss factor of said material, together with progressive dimensional and/or strength changes of the profile or portion. These factors should be borne in mind when subjecting the profile to the microwave radiation, by adjusting or tuning the microwave radiation so that, at each level or position along the length of the descending profile or portion, it is subjected to optimum or at least adequate microwave radiation, particularly as far as intensity/power is concerned, so that it is heated at an acceptable rate of temperature increase and undergoes such temperature holds or soaks at constant temperature for such periods as are acceptable or desirable. Routine experimentation should be employed to optimize the microwave radiation so that the profile or portion is at different temperatures at different positions along its length, for acceptable drying, calcining and sintering. Suitable drying regimes or methods are described in U.S. Pat. No. 5,979,073, assigned to Implico B. V. Typically, the microwave radiation to which the profile or portion is subjected will vary (increase or decrease) as regards at least some characteristics thereof along the length of the profile or portion, although it may naturally remain constant over selected portions of said length. Preferably, the profile is subjected to substantially uniform microwave radiation over its external surface area. It should be noted that to achieve optimum radiation of the profile, there should be no foreign or extraneous material, such as impurities present in the material forming the profile, present in the cavity which could interfere with the radiation being applied to the profile. Naturally, the support means supporting the profile should preferably be located at a position spaced or removed from the microwave cavity.
The ceramic product made in accordance with the invention may be a ceramic filtration support suitable for use in supporting a filtration membrane in a filter element used for micro- or ultrafiltration applications, particularly high pressure filtration. It will be appreciated that the process in accordance with the invention is not limited to filtration supports for filter elements, but extends to the making of other extruded ceramic products produced by extrusion, such as hollow extruded clay sections, clay bricks, clay tiles or the like.
It will be appreciated that, dependent upon the ceramic product required, the extrusion may be a paste selected and/or blended from one or more suitable ceramic materials or precursors thereof. Typically the following factors will be taken into account when selecting or blending a suitable ceramic material to be used in accordance with the process of the present invention:
Finished Product Attributes
Porosity PA0 Strength PA0 Impact resistance PA0 Abrasive wear resistance PA0 Chemical resistance PA0 Temperature shock resistance PA0 Operating temperature PA0 Thermal and electrical conductivity PA0 Expansion coefficient PA0 Ability to be efficiently blended PA0 Plasticity PA0 Cohesive strength PA0 Lubricity PA0 Repeatability of performance PA0 Drying behaviour PA0 Binder performance through the various temperature regimens PA0 Stability through changes of state PA0 Dielectric characteristics and conductive characteristics, eg dielectric constant and loss factor(s), throughout the heating cycle.
Extrusion Attributes
Drying/Sintering Attributes
When the extrusion is to be handled or supported at one or more intermediate stages of its treatment, additional criteria or attributes, eg strength in the intermediate state, will need to be considered when selecting or blending a suitable ceramic composition for extrusion.
In a particular embodiment of the invention a suitable green ceramic composition for a high grade filter support may comprise suitable proportions of aluminum oxide (ceramic), methyl cellulose (binder/plasticizer), polyalkylene glycol (lubricant), aluminium oxide monohydrate (plasticizer/sintering aid) and aluminium oxychloride resin (binder).
More particularly, when the process of the invention is used for making a filtration support, the extrusion may be of a particulate mixture comprising a suitable ceramic such as .alpha.-alumina or a precursor thereof, and of a maximum particle size of 100 .mu.m, preferably 60 .mu.m, and an average particle size of 10-40 .mu.m, preferably 15-25 .mu.m. The extrusion preferably takes place of a green paste material having a moisture content of 5-20% by mass, preferably 8-15% by mass. The green material may comprise an organic binder/plasticizer which makes up 1-20%, typically 2-4%, by mass of the mixture on a dry basis, an inorganic binder which makes up 2-10%, typically 4-6% by mass of the composition on a wet basis, a lubricant which makes up 0.5-3%, eg 0.7-1,2% on a wet basis and a plasticizer/sintering aid which makes up 2-10%, preferably 4-6% by mass on a wet basis. By a precursor is meant a material, which, when subjected successively to drying, calcining and sintering, is converted into the ultimate ceramic in question.
The particle size will be selected to provide a product of the desired porosity and pore size, while the moisture content, and type and proportion of binder, will be selected for suitable extrusion (lubrication of the die, green strength, etc) and for suitable microwave heating (loss factors, internal stresses in the profile or portion, homogeneous heating, etc).
Using a clay such as mullite as a binder allows low sintering temperatures of 1300-1400.degree. C., eg 1350.degree. C., whereas using organic and/or inorganic binders, plasticizers, lubricants and sintering aids allows a high purity ceramic (eg .alpha.-alumina) product to be produced, but usually requires higher sintering temperatures of &gt;1550.degree. C., eg the 1600-1800.degree. C. mentioned above.
For ceramic filtration supports the process may comprise extruding the profile which is usually circular (although it may be eg hexagonal or octagonal) in cross-section, and so that it has a plurality of parallel passages in its interior extending along its length, of a desired size and spacing. Thus, it may be extruded with a plurality of filtration passages which are arranged in eg concentric circles seen in end elevation, and with one or more drainage passages, the filtration passages typically being of the same or a smaller diameter than the drainage passage(s), of which there is conveniently one, centrally located in the profile. In this way a support can be provided which can typically be used by feeding a fluid to be filtered into one end thereof, into the filter passages under pressure, retentate passing out of the opposite end of the support and filtrate or permeate permeating transversely through the material of the support, to exude from its outer surface and/or into such drainage passage or passages as are provided, to drain from the ends thereof.
In this case the process will typically include the step of applying a filter membrane to the walls or surfaces of the filter passages, optionally after drying and cooling and before sintering, so that the membrane can be sintered to said passage surfaces in the sintering step. Suitable membranes can be applied in known fashion, eg using sol-gel technology, various examples being set out in R. R. Bhave. Inorganic Membranes: Synthesis, Characteristics and Applications. pp 21-38, Van Nostrand Reinhold, New York. A revolving or rectilinear pocketed receiver may be used to store the separated portions of profile, either before or after the sintering, or both, as required by the sequence of steps chosen. Naturally, if desired, the filter membrane may be applied after sintering, the support being then re-sintered to sinter the membrane in place.
In a particular embodiment of the process a profile is extruded which is of relatively large cross-section, eg equivalent to that of a circular profile having a diameter of 50-200 mm or more, and in lengths of over 1500 mm, eg 3000-6000 mm or more, before it is severed. It will be appreciated that the length of the profile will in principle be governed only by height of the installation, and by such difficulties, if any, as are encountered in supporting the profiles.
Although extrusion in a vertically downward direction has been emphasised hereinabove, it will be appreciated that, for suitable profiles, the process can be carried out in a vertically upward direction or a horizontal direction, eg using a walking beam or pusher system or the like, which supports the profiles and carries them through a microwave cavity, optionally intermittently, with microwave irradiation being discontinued during movement. In this way, eg by cutting the supported profile at an early stage into short lengths, tiles can be made from a flat extrusion, or ceramic filtration supports can be made from an elongated extrusion, or discs for use as plates can be cut from a cylindrical extrusion, before being carried through a microwave cavity.
Suitable microwave radiation can have a wavelength of 1-10 GHz, preferably 2.45 GHz, the intensity or power naturally being selected, depending on the composition, cross-section and rate of movement of the profile or portion and on the desired heating rate and depth of penetration required thereof, as well as capital costs.
According to another aspect of the invention there is provided an installation for use in the making of a dried sintered ceramic product, which installation comprises:
The installation may include severing means for severing the profile into a plurality of portions, after drying of the profile.
The extrusion means may be a ram- or a screw extruder provided with a suitable die; and it may be arranged to effect vertical extrusion of an elongated profile in a downward direction. Preferably, the extrusion means is an extruder, such as a screw extruder, provided with a die, the extruder being arranged to effect extrusion of an elongated profile in a vertically downward direction.
The support means may comprise one or more chucks, or friction support means, arranged to move, as described above, substantially synchronously with the elongated profile as it is extruded. Accordingly, the support means may comprise at least one chuck arranged to grip the profile, each chuck being arranged, while gripping the profile, to move downwardly synchronously with the profile as the profile is extruded. The support means may be arranged to support the elongated profile at various points along its length so that it is subjected to tensile and compressive stresses capable of being withstood by said profile, bearing in mind its strength and other physical characteristics at the various stages of the microwave heating.
There may be a plurality of microwave radiation sources, and at least some said sources may be adjustable, eg as to power output (amplitude/intensity) and/or wavelength/frequency.
The microwave radiation source or sources may be coupled into at least one of a travelling-wave, dynamically tuned standing-wave, meander- or serpentine, multimode applicator or any other suitable microwave applicator. In a preferred embodiment for the purposes of drying and/or sintering large-diameter ceramic extrusions, a multimode cavity may be employed to generate overlapping standing waves. The microwave heat sources may be arranged in a microwave cavity which is located below the extruder so that the cavity can receive the profile as it is extruded, the or each support means being located below the microwave cavity together with the severing means. Preferably, the size and shape of the cavity is arranged to enclose the profile in a snug or close-fashion.
Preferably, the installation has a microwave cavity, the cavity being provided with a plurality of said microwave sources, at least some of which are adjustable as regards their microwave output, the sources being vertically and circumferentially spaced from one another around a central zone in the cavity down which, in use, the profile passes. The radiation sources may form part of a multimode microwave cavity.
The severing means may be a flying cut-off saw such as a diamond saw, a water nozzle, a laser, or the like, whichever is suitable for the purpose and/or depending on the ceramic temperature, etc.
Optionally, the installation includes a storage/cooling device, eg a revolving pocketed receiver.
The installation may comprise two microwave cavities, namely an initial cavity, provided with the extruder as described above and with the severing device, for drying and/or calcining the profile, and a final cavity, provided with support means for lowering severed portions of the profile, vertically aligned, therethrough, for sintering the portions. A said storage and cooling device may be provided between the cavities and after the final cavity. Naturally, drying and sintering may, if desired, be carried out sequentially in a single cavity.
The microwave sources may be arranged in each cavity, in circumferentially and/or longitudinally spaced relationship, around a central path in the cavity down which the profile or portion, as the case may be, passes, the sources being arranged to direct microwave radiation at a said profile or portion in said path. Each microwave cavity may be provided by a vertically extending microwave oven having an outlet at its lower end and the extruder or an inlet at its upper end, as the case may be.
The invention also extends to a dried profile whenever prepared by a process as described above.
As mentioned above, the profile may have a plurality of parallel passages in its interior extending along its length, the profile being circular in cross-section and having a diameter of 50-200 mm, and the profile having a length of 300-6000 mm.